1. Field of the Invention
The present invention relates to a process for forming an amorphous or crystalline functional film for a semiconductor, insulator, conductor, etc., and in particular a deposited film adapted for use in an active or passive semiconductor device, an optical semiconductor device, a solar cell or an electrophotographic photosensitive device.
2. Description of the Prior Art
A deposited film can be obtained by various processes such as vacuum evaporation, plasma CVD, thermal CVD, photo CVD, reactive sputtering, ion plating etc., among which the plasma CVD process is widely employed for commercial purpose.
However, for application in electronic or optoelectronic devices of higher performance, the deposited films obtained with such processes still have a room for improvements in overall characteristics, including electrical and optical properties, resistance fatigue due to environmental conditions and repeated usage. Further, improvement in productivity and mass productivity including uniformity and reproducibility is desired.
The reaction process in the film formation by the plasma CVD process conventionally employed is considerably more complex than that in the so-called thermal CVD process and is not yet clarified in detail. Also the formation of the deposited film depends on a number of parameters such as the substrate temperature, flow rates and ratio of introduced gasses, pressure at the film formation, high-frequency electric power, electrode structure, reactor structure, gas exhaust rate, plasma generating method etc., which, in combination, may give rise to unstable plasma, thus often resulting in significant undesirable affects on the deposited film. Besides it has been difficult to uniformalize the manufacturing conditions since certain parameters have to be selected for each apparatus employed.
For the preparation of an amorphous silicon film, the plasma CVD process is currently regarded as best, since it can provide electrical and optical properties at levels satisfactorily acceptable in various applications.
However, for achieving reproducible mass production of a film of a large area with uniform thickness and quality for certain applications, the plasma CVD process requires a significant investment in the apparatus, involving complex controls, tight tolerances, and subtle adjustments. These matters have been pointed out as the problems to be improved in the future.
On the other hand, the conventional CVD technology requires the use of a high temperature, and still is not necessarily capable, on a commercial scale, of providing a deposited film with satisfactory characteristics.
These unsolved problems are particularly marked in preparing a thin film of compounds of group II-IV and group III-V elements.
For these reasons there has been desired a process for forming a functional film in an inexpensive apparatus, while maintaining acceptable characteristics and uniformity in practice.